Four charges equal to $-Q$ are placed at the four corners of a square and a charge $q$ is at its centre. If the system is in equilibrium the value of $q$ is

• A

  $-\frac{Q}{4}(1+2 \sqrt{2})$
• Correct answer
  B

  $-\frac{Q}{2}(1+2 \sqrt{2})$
• C

  $-\frac{Q}{2}(1+2 \sqrt{2})$
• D

  $\frac{Q}{2}(1+2 \sqrt{2})$

Hence, the correct answer is option: 'B'.